Two types of gas turbine engines currently available to power aircraft are the turbo-fan and turbo-prop engines. Common to both engines is the power generating unit, commonly referred to as the core engine. This unit typically includes a compressor section, a combustor, and a turbine section in serial flow relationship. Pressurized air from the compressor section is mixed with fuel and burned in the combustor to produce a high energy gas stream. The gas stream expands through a first turbine section where energy is extracted to operate the compressor. Such engines further include a second turbine, known as a power turbine, located downstream (aft) of the first turbine section. The power turbine extracts energy from the gas stream to power propulsor blades such as a fan or propeller.
The highest temperatures in the engine are those in the combustor and turbines. Any components of the engine which are located near these high temperature regions and which may be damaged by excessive heat must be cooled. Pressurized air for cooling these components is typically obtained from the compressor, fan duct, or otherwise drawn in from the atmosphere.
In most fan or propeller driven engines, the propulsor blades are located generally forward of the core engine. In such applications, the blade hub structures (base of the blades) operate in a relatively low temperature environment obviating the need for cooling the hub structures.
A recent improvement over the turbo-fan and turbo-prop engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 071,594--Johnson, filed July 10, 1987. In the unducted fan engine, the power turbine includes counterrotating unducted fan blades. The fan blades are generally variable pitch blades to achieve optimum efficiency from the engine. To vary the pitch of each blade, each blade hub structure includes a bearing or other anti-friction coupling. If the engine has variable pitched blades, it must have a mechanism for varying the pitch of the blades. Blade pitch varying mechanisms located adjacent to each blade hub have been disclosed in U.S. Pat. No. 4,738,591, issued Apr. 19, 1988. Johnson discloses the location of the fan blades (propulsor blades) generally aft of the core engine and radially outwardly of the power turbine section. Because of the close proximity of the fan blades to the power turbine in such a configuration, the blade hub structures, under certain flight conditions, will be subjected to relatively high rates of heating (heat loads).
The air temperatures in the hub region, i.e., the region in the power turbine near the base of each blade, will vary depending upon flight conditions. For example, during periods of relatively high power demand, such as during take-off, turbine and combustor temperatures are elevated resulting in higher blade hub region temperatures. Blade hub structures and pitch varying mechanisms are generally made of lightweight cost effective materials. Such materials generally have relatively low temperature limits. Consequently, more cooling of the hub regions may be required during such high power take-off conditions than is normally required during cruise conditions. Increased ventilation of the blade hub region may also be beneficial or required during idle and reverse thrust operational even though the heat load is generally lower than take-off conditions. In contrast, temperatures stabilize at a lower level during steady state cruise operating conditions and less cooling is required. Since any cooling system will have a performance penalty associated with its use, it is advantageous to provide cooling only at the level required. Thus, means for automatically varying the amount of cooling air to the hub region of such blades is desired.
It is possible that some of the components in the hub region may be more sensitive to high temperatures than the other component. For example, hydraulic components of a pitch varying mechanism might not be able to withstand as great a temperature as the hub. Therefore, it may be desirable to provide more cooling to some components than to others.
Accordingly, it is an object of the present invention to provide an improved ventilation system for a power turbine section of a gas turbine engine.
It is another object of the invention to provide a ventilation system for a propulsor blade hub region within an unducted fan gas turbine engine.
It is another object of the present invention to provide an automatic ventilation system for controlling ventilation of the hub region of an unducted fan engine.
It is a further object of the present invention to provide a ventilation system for an unducted fan engine which apportions the ventilation to different locations.